1. Field of the Invention
The present invention relates to a method suitable for wavelength division multiplexing, and more particularly to a method, device, and system for level equalization.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and a pumping unit for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has been developed to amplify signal light having a wavelength band of 1.55 xcexcm where the loss in a silica fiber is minimum. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 xcexcm band or a 1.48 xcexcm band, a gain band including a wavelength band of 1.55 xcexcm can be obtained.
Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain main signal light (WDM signal light), which is output to an optical fiber transmission line. At a receiving end, the main signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data (a main signal) is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity by a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain of the optical amplifier which characteristic is represented by a gain deviation or gain tilt. For example, in a typical EDFA, it is known that a gain deviation is produced at wavelengths near 1.55 xcexcm. If the gain deviations in a plurality of cascaded EDFAs are accumulated, an optical SNR (signal-to-noise ratio) in a channel included in a low-gain band is degraded. Accordingly, to allow high-quality transmission, it is preferable to flatten the wavelength characteristic of gain of an optical amplifier.
A long-haul repeatered transmission system including an optical fiber transmission line for transmitting WDM signal light and a plurality of optical repeaters arranged along the optical fiber transmission line is in practical use. Each optical repeater includes an EDFA for compensating for loss of the optical fiber transmission line. Further, a gain equalizing filter is adopted to equalize the wavelength characteristic of gain of the EDFA. However, it is difficult from the viewpoint of fabrication technique to make the wavelength characteristic of gain of the EDFA precisely coincide with the wavelength characteristic of loss of the gain equalizing filter, causing an equalization residual deviation. In the case of multirepeatered transmission, the equalization residual deviation is accumulated to incur an unneglible interchannel level deviation, causing a degradation in transmission quality. This is a principal factor that limits a transmittable distance and a transmittable wavelength band.
On the other hand, there is a possibility that the transmission line loss may change during operation of a system because of aged deterioration of the optical fiber transmission line or repair against a breakage accident of the optical fiber transmission line. To cope with this, each optical repeater performs output maintaining control such that an output level is maintained constant. For example, this control is such that the output level is monitored and the power of pump light to be supplied to the EDFA is controlled according to the result of this monitoring. However, when the power of the pump light for the EDFA is changed for the output maintaining control, the gain of the EDFA changes to result in a change in wavelength characteristic of gain. If this change in wavelength characteristic of gain is accumulated over the length of the transmission line, the transmittable distance and the transmittable wavelength band are limited as similar to the above.
For relaxation of these limits, it is necessary to repair the interchannel level deviation at suitable intervals in the transmission line and thereby reduce the interchannel level deviation at a receiving end. For example, a system in practical use employs a method such that a gain equalizing filter for compensating for the accumulation of an equalization residual deviation is inserted in the transmission line at every tens of repeaters. In this method, however, the wavelength characteristics of gain of tens of repeaters are measured at a site where an optical repeater and an optical fiber each provided as a product are connected, and a suitable one is selected from a plurality of gain equalizing filters according to the result of this measuring. Thus, this method is very complex to perform.
It is therefore an object of the present invention to provide a simple method, device, and system for allowing automatic level equalization. Other objects of the present invention will become apparent from the following description.
In accordance with a first aspect of the present invention, there is provided a method for level equalization. First, a first optical amplifier having a gain tilt coefficient defined as a change in gain tilt to a unit change in gain is provided. Second, a second optical amplifier having a gain tilt coefficient different from the gain tilt coefficient of the first optical amplifier is cascaded to the first optical amplifier. Then, the gains of the first and second optical amplifiers are distributed so that a level tilt of light amplified by the first and second optical amplifiers becomes substantially flat.
Thus, it is possible to provide a simple method for allowing automatic level equalization by providing a plurality of optical amplifiers having different gain tilt coefficients and distributing the gains of these optical amplifiers so that a level tilt of light amplified by these optical amplifiers becomes substantially flat.
In accordance with a second aspect of the present invention, there is provided a device comprising first and second optical amplifiers and a control unit. The first optical amplifier has a gain tilt coefficient defined as a change in gain tilt to a unit change in gain. The second optical amplifier is cascaded to the first optical amplifier and has a gain tilt coefficient different from the gain tilt coefficient of the first optical amplifier. The control unit distributes the gains of the first and second optical amplifiers so that a level tilt of light amplified by the first and second optical amplifiers becomes substantially flat.
Thus, it is possible to provide a simple device for allowing automatic level equalization by cascading a plurality of optical amplifiers having different gain tilt coefficients and distributing the gains of these optical amplifiers so that a level tilt of light amplified by these optical amplifiers becomes flat.
In accordance with a third aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths, and a plurality of optical repeaters arranged along the optical fiber transmission line. At least one of the plurality of optical repeaters includes the device according to the present invention.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.